The present invention relates to a braking system for trailers of utility vehicles which have a steerable front axle, including front axle brake cylinders, rear axle brake cylinders, rotational wheel speed sensors and an ABS valve.
From German Patent Document DE 38 29 951 C2, a utility vehicle braking system for the load-dependent brake pressure control is known in which a solenoid valve, corresponding to a conventional ABS valve, as well as a rotational wheel speed sensor are assigned to each of the brake cylinders of the four vehicle wheels. The solenoid valves and the rotational wheel speed sensors are connected with a central control unit, which controls the solenoid valves during braking as a function of the axle load distribution. In the case of this system, there is no determination at all of the absolute wheel slips for the axle-load-dependent brake pressure control. On the contrary, for the analysis of the rotational wheel speed signals, the exceeding of a rather rough interaxle “rotational wheel speed difference threshold” is, in each case, used as a criterion for the brake pressure limitation.
From German Patent Document DE 44 43 522 A1, a method is known for determining the incline of a road by which, for a powered driving condition and a free-rolling driving condition, in each case, a rotational wheel speed difference of a powered axle and of a non-powered axle is determined and, from these differences, an incline constant is determined.
From German Patent Document DE 198 09 546 C1, a method for automatically determining the wheel base of steerable vehicles during cornering is known, in which case the wheel base is determined from a defined wheel track and measured wheel circumference speeds.
From the applicant's internal state of the art, an electronic braking system for utility vehicle trailers is known, in which an active pressure control module is, in each case, assigned to the brakes of the front axle and the brakes of the rear axle. These pressure control modules each have three solenoid valves, specifically for ventilating and bleeding, and for a “back-up circuit”, as well as a pressure sensor. The controlling of the pressure control module of the front axle takes place by way of the pressure control module of the rear axle, which leads to high wiring expenditures. Specifically, three control lines and one grounding conductor for the solenoid valves, as well as a sensor line and a grounding conductor for the pressure sensor, are required for this purpose.
It is an object of the present invention to provide a braking system for utility vehicle trailers which is cost-effective and requires lower constructional expenditures.
This object is achieved by providing a braking system for utility vehicle trailers which have a steerable front axle, including front axle brake cylinders, rear axle brake cylinders, rotational wheel speed sensors and an ABS valve. The ABS valve is assigned jointly to both brake cylinders and is provided for controlling brake pressure into the front axle brake cylinders. An EBS module is assigned to the rear axle brake cylinders for controlling brake pressure into the rear axle brake cylinders. An electric control output of the EBS module is connected with an electric control input of the ABS valve. The rotational wheel speed sensors are connected to the EBS module for determining a differential slip between the front axle and the rear axle. The EBS module controls the ABS valve as a function of the determined differential slip. Advantageous embodiments and further developments of the invention are contained in the subclaims.
It is the main principle of the invention to control the brakes of the front axle of the utility vehicle trailer by a single joint ABS valve, which is assigned to the two front axle brakes and which is controlled by an electronic braking system, that is, by an EBS module, primarily assigned to the rear axle brakes, specifically as a function of a so-called “differential slip” between the front axle and the rear axle.
More simply stated, the brake pressure control of the rear axle brakes takes place by means of a pressure control module which electrically controls the ABS valve assigned to the two front axle brakes.
The ABS relay valve consists only of two solenoid valves for the holding or bleeding of brake pressure. A pressure sensor for the front axle brakes is not required here. A back-up valve is also not provided. Thus, for connecting the pressure control module with the ABS valve, only three connection lines are required.
Since, according to the invention, no pressure sensor is provided at the front axle, it is true that the pressure control at the front axle brakes cannot be implemented quite as precisely as by means of a pressure sensor. However, the “absence” of a pressure sensor at the front axle can largely be compensated by a so-called “differential slip control algorithm” because the wheel speeds are measured and wheel slips at the wheels of both axles are determined therefrom, which permits the obtaining of “information” concerning the momentary braking power distribution.
The brake pressure control at the front axle therefore takes place as a function of the rotational wheel speed signals supplied by rotational wheel speed sensors and a “differential slip signal” between the front axle and the rear axle derived therefrom.
The object of the control consists of controlling the front axle brake pressure such that the slip occurring during a braking at the front wheels and the rear wheels is identical, or that the difference between a “slip signal” of the front axle and of the rear axle is kept in a defined value range.
In this manner, an approximately uniform deceleration of both axles or a uniform deceleration ratio is ensured. For this purpose, the front axle brake pressure is modulated correspondingly; that is, the brake pressure defined by way of the pneumatic control line is reduced, as required. The reason is that the brake pressure defined by way of the pneumatic control line corresponds to the brake pressure which can be maximally controlled by way of the ABS valve into the front axle brake cylinders.